This invention relates generally to the field of optical communications and in particular to a method and apparatus for producing a flat Raman gain over very long bands.
Early proposals for all-optical transmission (see, e.g., A. Hasegawa, xe2x80x9cNumerical study of optical soliton transmission amplified periodically by the stimulated Raman process,xe2x80x9d Appl. Opt., 23,1095 (1984); L. F. Mollenauer, J. P. Gordon, and M. N. Islam, xe2x80x9cSoliton propagation in long fibers with periodically compensated loss,xe2x80x9d IEEE J. Quantum Electronics QE-22, 157 (1986)) were based on the use of gain from the Raman effect to turn spans of transmission fiber into their own amplifiers. The scheme afforded many fundamental advantages, and indeed, it was used successfully for the first demonstration of an all-optical, long-distance transmission and subsequently reported by L. F. Mollenauer and K. Smith, in an article entitled xe2x80x9cDemonstration of soliton transmission over more than 4000 km in fiber with loss periodically compensated by Raman gain,xe2x80x9d which appeared in Opt. Lett. 13, 675 (1988).
Nevertheless, with the advent of the erbium fiber amplifier in the late 1980""s, Raman amplification temporarily fell out of vogue, largely due to the pump powers required. That is, within the context of single-channel transmission, where the signal powers are rarely more than one or two milliwatts, the several hundreds of milliwatts threshold power required for net positive Raman gain seemed excessive and, at the time, impractical. With the recent ascendance of dense WDM (where net signal levels can easily reach one hundred mW or more), and with the simultaneous commercial availability of several-hundred mW output semiconductor pump lasers, however, opinion has changed. Now Raman gain is highly prized for its ability to overcome noise/non-linear penalties, and for the fact that the position and extent of the gain band depend only on the available pump wavelengths. Another extremely important advantage of Raman amplification for dense WDM lies in the fact that, in stark contrast to erbium amplifiers, the shape of the Raman gain band is essentially independent of pump and signal levels.
As a result of its importance to optical transmission systems, methods and apparatus which facilitate the production and utilization of Raman gain are desired and a continuous avenue for exploration.
We have developed a method and apparatus for producing a flat gain over very broad gain bands utilizing backward-pumped Raman amplification. The method allows for dynamic gain control through simple electronic means.
The method involves the time division multiplexing of combined pump wavelengths to attain broad Raman gain bands. Originally conceived as a way to prevent the various pump wavelengths from interacting with each other, our method has proven to have several other very great and important advantages, especially in a preferred, frequency-swept embodiment.
Specifically, our frequency-swept method produces extremely flat gain (variation less than 0.05%) across gain bands at least 8 THz wide while allowing for a wide variation of adjustment in the shape of the gain band, as might be required to overcome various system defects. Advantageously, all of these conditions may be established and altered within microseconds, utilizing known, simple, all-electronic control.